Is a Health Care POA from WV valid if my MIL currently resides in the state of PA?

Although it might be valid state-to-state, a lawyer friend suggested it's not a bad idea to get one in each state, if you can, just in case.

It should be valid in any state. Be very careful what you sign, though. Always write POA after your name. PA is famous for nursing homes that go after families to pay for care.
That POA does not mean you can sign her out of there. If she was remanded to the facility by court order, she may already be a ward of the state. I'm not really sure what you want to change.

Yes, a POA is valid from state to state whether the person with the POA moves or the person for whom you are POA moves.